IN EP 0 344 421 A1 a cermet is proposed which has either an average grain size of the hard material phase in the surface layer with respect to a core with a penetration depth of 0.05 mm, which is between 0.8 to 1.2 times the average grain size of the hard material phase in the cermet core, or at the same penetration depth relates to a binder phase which corresponds to 0.7 to 1.2 times the average binder content of the cermet core, or wherein the hardness in the mentioned penetration depth ranges between 0.95 and 1.1 times the average hardness of the cermet core. In order to produce this cermet, the starting mixture is sintered after grinding, blending and precompression, whereby in a first stage the sintering takes place at 1300 degree C or below under vacuum or in an inert gas atmosphere, while in a second stage the sintering takes place above 1300.degree. C. at a nitrogen pressure of 0.1 to 20 torr and whereby the nitrogen pressure is supposed to increase with the increasing temperature. The cooling also takes place in the presence of nitrogen.
The EP 0 368 336 B1 describes a cermet substrate with a hard surface layer, wherein the region with the maximal hardness lies at a depths between 5 .mu.m and 50 .mu.m from the substrate surface, and the substrate surface has a hardness of 20 to 90% in relation to the maximal hardness. In order to produce this cermet, the precompressed mixture is subjected to an initial temperature increase up to 1100.degree. C. in vacuum, to a subsequent temperature increase from 1100.degree. C. to a temperature range between 1400.degree. C. and 1500.degree. C. in a nitrogen atmosphere and to a final sintering in vacuum.
The EP 0 374 358 B1 describes a process for the production of a cermet with 7 to 20% by weight binder phase and a hard phase of titanium carbide, titanium nitride and/or titanium carbonitride with 35 to 59% by weight Ti, 9 to 29% by weight W, 0.4 to 3.5% by weight Mo, 4 to 24% by weight of at least one metal among Ta, Nb, V and zirconium, 5.5. to 9,5% by weight N.sub.2 and 4.5 to 12% by weight C. The formulated mass, blended, dried and precompressed, is sintered in such a manner that the temperature is increased to 1350.degree. C. in a vacuum, whereby the nitrogen atmosphere is set to 1 torr at 1350.degree. C., the partial nitrogen pressure being gradually increased together with a temperature increase from 1350.degree. C. up to the sintering temperature, whereby the nitrogen atmosphere is set at 5 torr when the sintering temperature is reached.
The EP 0 492 059 A3 describes a cermet body, whose hardness at a penetration depth of no less than 1 mm is higher than in the cermet interior, whereby the binder proportion can be reduced in a layer thickness of 0.5 to 3 .mu.m when compared to the core substrate. The cermet should have a hard material coating with a thickness of 0.5 to 20 .mu.m of carbides, nitrides, oxides and borides of titanium and Al.sub.2 O.sub.3. In order to produce this body, a green compact is preheated at first under vacuum to a temperature between 1100.degree. C. and 1400.degree. C., subsequently nitrogen gas is introduced up to a pressure wherein the partial nitrogen pressure ranges between 5 and 10 torr, so that the nitrogen is removed from the substrate surface. The sintering and the final cooling are performed in a nonoxidizing atmosphere, such as a vacuum or in an atmosphere of inert gas. Finally the body is coated by CVD or PVD.
For the production of a cermet with a high degree of toughness the EP 0 499 223 A1 proposes to set the relative concentration of the binder in layer close to the surface with a thickness of 10 .mu.m to 5 to 15% of the average content of binder in the cermet core, and in the layer underlying that of 10 .mu.m to 100 .mu.m penetration depth to set the binder content to 70 to 100% in relation to the cermet core, whereby compressive strains of 30 kgf/mm.sup.2 and more exits at the surface. In the thereby used process the sintering is performed in the presence of nitrogen with a constant pressure of 5 to 30 torr and the cooling takes place under vacuum with a cooling rate of 10.degree. to 20.degree. C. per minute.
On the other hand, the EP 0 515 340 A3 describes a cermet with zone close to the surface enriched with binder.
The EP 0 519 895 A1 discloses a cermet with a triple-layered rim zone, wherein the first layer reaches to a depth of 50 .mu.m and is rich in TiN, the next layer with a penetration depth of 50 to 150 um is built with a binder enrichment and the next layer between 150 .mu.m and 400 .mu.m is poor on binder compared with the inner cermet core. For this purpose the sintered body is treated in an atmosphere of N.sub.2 and or NH.sub.3, optionally in combination with CH.sub.4, CO, CO.sub.2 at 1100.degree. C. to 1350.degree. C. during one to 25 hours, at atmospheric pressure or a pressure above 1.1 bar.
The cermets known to the state of the art have either various binder contents at the surface, which can be recognized by their spotty appearance, or have a tendency of attachment of the binder to the sintered substrate, which leads to changes of the composition in the contact zone because of the reactions related thereto. Further disadvantages of the cermets known to the present state of the art are a partially high surface roughness, as well as poor attachment of the applied wear-resistant layers due to the increased binder content in the surface. As far as increased nickel contents appear in the surface, no CVD coating is possible. The mentioned disadvantages show particularly clearly that the cermets can not be used as cutting inserts in machining processes.